public class ObjectAssertions<TSubject, TAssertions> : ReferenceTypeAssertions<TSubject, TAssertions>
    where TAssertions : ObjectAssertions<TSubject, TAssertions>
{
    public AndConstraint<TAssertions> BeOneOf<TExpectation>(IEnumerable<TExpectation> validValues, IEqualityComparer<TExpectation> comparer, string because = "", params object[] becauseArgs);
}

var obj = new SomeClass(1);
var expected = new[] { new SomeClass(1), new SomeClass(2), new SomeClass(3) };
obj .Should().BeOneOf(expected, new SomeClassEqualityComparer ());

internal class SomeClass
{
    public SomeClass(int key)
    {
        Key = key;
    }

    public int Key { get; }

    public override string ToString()
    {
        return $"SomeClass({Key})";
    }
}

internal class SomeClassEqualityComparer : IEqualityComparer<SomeClass>
{
    public bool Equals(SomeClass x, SomeClass y)
    {
        if (ReferenceEquals(x, y))
        {
            return true;
        }

        if (ReferenceEquals(x, null))
        {
            return false;
        }

        if (ReferenceEquals(y, null))
        {
            return false;
        }

        if (x.GetType() != y.GetType())
        {
            return false;
        }

        return x.Key == y.Key;
    }

    public int GetHashCode(SomeClass obj)
    {
        return obj.Key;
    }
}